Harnessing interfacial hydrogen bonds to modulate hydrogen evolution and oxidation reaction kinetics

Controlling electrochemical reactions such as proton-coupled electron transfer reactions requires a fundamental understanding of solvation environments at the electrode-electrolyte interface. While electrolyte compositions have been shown to significantly alter the catalytic activity, the mechanistic role of the electrolytes and their structures at the electrochemical double-layer remains poorly understood. Here, we systematically tuned the water structures through highly concentrated water-in-organics electrolytes, where we confined water in aprotic organic solvents. Decreasing the water-to-organic solvent ratio, water molecules became more isolated and showed increased pKa, which is correlated with more negative water reduction potential on polycrystalline Pt. The hydrogen evolution/oxidation (HER/HOR) reaction kinetics were solvent-dependent, where electrolytes containing organic solvents with high donor numbers, such as dimethyl sulfoxide (DMSO), decreased the HER activity more than the electrolytes with low donor number solvents, such as acetonitrile (ACN). DMSO promoted isolated water at the interface through stronger intermolecular hydrogen bonds between individual water and DMSO, while ACN promoted more symmetric H-bonded water at the interface. Further support for the proposed mechanism came from in situ surface-enhanced infrared absorption spectroscopy (SEIRAS) measurements and molecular dynamics (MD) simulations. Decreasing HER activity can be correlated to the increasing fraction of isolated water near the electrical double layer. Such molecular-level understanding could facilitate electrolyte design by modulating non-covalent hydrogen bonding interactions at the electrified interface to control the kinetics of HER and other proton-coupled electron transfer reactions.